SEMAEOSTOME^^ — PELAGIA. 573 



when expanded. In ordinary contraction it is about 49 to 55 mm. in diameter and 31 mm. 

 high. Sides of bell relatively straight and sloping, the apex flat. Numerous nettle-warts 

 over the exumbrella, arranged in more or less irregular lines radiating from aboral apex of 

 exumbrella. These warts are rich orange-red in color and are elongate and linear, some- 

 times with, but more often without, cross-foldings. Near the bell-margin, however, they 

 lose their linear shape and become small, simple, and more or less oval. 



The 8 marginal sense-organs are set in deep niches in the perradii and interradii. The 

 sense-club has no ocellus, but contains only a terminal mass of deeply pigmented orange-colored 

 crystalline concretions of entodermal origin. There is no sensory pit in the exumbrella above 

 the sense-club. The 8 hollow, tapering tentacles are each about twice (115 mm.) as long as 

 the bell-diameter. 



There are 16 subrectangular marginal lappets, with shallow median notches and rounded 

 angles. The septum between the ultimate branches of the radiating stomach-pouches in the 

 marginal lappets is twice as wide as the ultimate pouches themselves. The 4-sided throat- 

 tube is as long as the bell-radius. The 4 lanceolate lips or palps, with their complexly folded 

 margins are each about 1.33 as long as the bell-diameter. Thus in an adult medusa with a 

 disk 49 mm. wide the palps were 68 mm. long. 



The bell has a rich rose-purple tinge; the gonads, the entodermal cores and the ten- 

 tacles being especially deep in this color. The warts upon the exumbrella and along the 

 outer edges of the palps are orange-brownish red. 



This medusa is abundant at times in the Mediterranean especially in summer, although 

 large specimens are rarely seen in winter. It may be locally abundant during several suc- 

 cessive seasons and then vanish for years. For many years it was all but unknown in the 

 Bay of Naples but since 1900 it has been one of the commonest Sc) phomedusse in this 

 region. It ranges widely over the warm regions of the Atlantic. 



The development has been studied by Krohn, Kowalevsky, Hamann, Goette, Hyde, and 

 Metschnikoff. Hamann, 1883, has made a detailed study of the development of the gonads, 

 and their structure has been described by the brothers Hertwig, 1878. They appear as 4 

 interradial, elongate ridges in the entoderm of the subumbrella. The entoderm forms a series 

 of follicles in which the sex-cells develop and then migrate into a gelatinous lamella between 

 the layers of entoderm. 



According to Metschnikoff, the egg is violet-brown and is laid between 12 and 2 in the 

 afternoon, in December, in the Mediterranean. Segmentation is total and nearly equal, 

 and a very large central segmentation-cavity is formed. The gastrula results from invagination 

 at the hinder end of the body. The blastopore does not close, but forms the mouth of the 

 larva. Thus, according to Goette, 1893, the mouth is ectodermal and forms by invagination 

 at the hinder end of the larva, but the invaginated sac by no means fills the segmentation 

 cavity. The first pair of stomach-pouches arise from the entoderm and are 180° apart, then 

 follows an ectodermal pair 90° away from the first. The latter then develop 2 lateral pouches 

 each, and at a later period the entodermal pair each gives rise to 2 lateral pouches, thus giving 

 a larva with 6 ectodermal and 6 entodermal stomach-pouches; finally the ectodermal pouches 

 give rise to 4 new adradial pouches and the larva has 16 stomach-pouches — 10 ectodermal 

 and 6 entodermal. There is thus a striking analogy between its development and that of 

 the scyphostoma of Aurellia, according to Goette. 



The external features of the transformation of the free-swimming larva into the medusa 

 have been studied by Krohn (1855), Kowalevsky (1873), etc. The mouth-end of the larva 

 becomes expanded and crater-like, with the mouth at summit of central cone of crater. The 

 depressed region around the cone becomes the subumbrella. The lappets, into which the 

 gastrovascular cavity is continued, grow out at intervals around the margin. The covering 

 of cilia is lost from the body of the larva and it begins to swim by means of rhythmical con- 

 tractions of its oral disk. Thus the free-swimming scyphostoma is converted into a medusa 

 without strobilization (see Goette, 1893.) 



Reasoning by analogy from the excellent work of Hyde, 1894 (Zeit. fur wissen. Zool., Bd. 

 58, p. 531), upon Aurellia, it is probable that only the subumbrella floor of the second pair 

 of evaginated gastric pouches is formed from ectoderm, their exumbrella sides being of 

 entoderm. (See also Hadzi's work upon Chrysaora.) 



